Genus Chara: Useful Notes on Genus Chara (2471 Words) | Biology

Here are your useful notes on Genus Chara!

There are about 90 species in this genus.

Occurrence:

The species of Chara grow submerged attached to the muddy bottom of the pools and ponds of clear water.

They prefer less oxygenated and hard water. The species of Chara are not found in the waters where mosquito larvae are present. They form sub-aquatic meadows extending to miles.

The plants of Chara are of great ecological value. As they are covered with calcium carbonate deposits, they deposit lot of calcium in the bottom of lake, etc., and after a considerable time the whole lake or pond is filled up with calcareous deposites.

Image Curtsey: http://ccala.butbn.cas.cz/col_images/285.jpg

Vegetative structure:

The plant is always found attached to the substratum by a well developed rhizoidal system. The rhizoids are uniseriately branched and obliquely septate. The rhizoids may or may not be differentiated into nodes and internodes.

The axis has nodes and internodes. Several branches of limited growth also known as leaves grow in whorls from the nodes of the axis. These leaves do not grow further after attaining a definite length. The leaves may or may not be differentiated into nodes and internodes. They may be branehed or simple. This ineternodal cell is sufficiently long and ensheathed by a cortex of vertically elongated cells. The cortex is one celled in thickness and consists of the cells of lesser diameter.

Apical growth:

The apical growth of the axis takes place by the terminal dome-shaped cell which cuts other derivatives at its posterior face. The derivative of the apical cell divides by a transverse wall producing two daughter cells. The upper daughter cell is the nodal initial and the lower one the internodal initial.

The lower cell (internodal initial) does not divide further and elongates several times of its original length to give rise to an internode. The upper cell (nodal initial) divides by a vertical wall producing two daughter cells. These daughter cells again divide vertically in such a way that two cells remain in the centre called central cells usually surrounded by 6 to 20 peripheral cells.

The central cells may or may not divide further. The peripheral cells divide periclinally. In such a way each peripheral cell divides into two cells. The outer one acts as leaf initial and the inner as nodal which may or may not divide further, if divides the division is vertical.

Development of cortex:

The peripheral cells of the nodal regions of the leaves never become apical cells but they may produce one-celled branch known as stipules, but this is not necessary for basal node of each leaf. On adaxial side, the upper and lower cells of the basal node protrude. After protrusion a transverse septum is formed at the base of these cells, thus initiating the cortical initials of corticating filaments. This way one of the cortical initials develops towards upper side and the other towards lower side.

These cortical initials act as apical cells and give rise to ascending and descending cortical filaments respectively ensheathing the main axis. In young corticating filaments both the nodal and internodal cells are approximately of equal size, but later on the internodal cell and the two peripheral nodal cells elongate sufficiently. The central cell of the nodal region does not elongate. Sometimes one celled spines develop from this central cell. (See Figs. 4A.4 and 4A.7).

Development of leaf:

As already mentioned the peripheral cells of the nodal region divide periclinally. The outer daughter cell acts as leaf initial and a leaf develops from it. The structure of the leaf resembles that of axis. The apical cell of the leaf cuts the derivatives in the same way as the main axis. 5-15 derivatives are cut from the apical cell.

The first derivative becomes the leafs basal cell. Rest of derivatives divide and give rise to an internodal and nodal initial. The internodal initial gives rise to a comparatively shorter intemode than that of axis. The peripheral cells of the nodal region of the leaf do not give rise to the leaves as in the case of axis. Here the peripheral cell gives rise to a single celled structure called the ‘stipule’. The cortex initials found on the leaf give rise to corticating filaments in the same way as on the axis.

Origin of rhizoids:

The rhizoids arise from the buried parts of the nodes. The peripheral cells of buried nodes give rise to rhizoids. The rhizoids possess oblique septa. Sometimes the upper and lower parts of the rhizoid at the septum protrude upward and downward in prolonged cells. Each of such prolonged cells may divide further and give rise to secondary rhizoids.

Structure of the cell-nodal cell:

There is no large central vacuole. The dense cytoplasm is filled up in the cell. There are numerous discoid chloroplasts, devoid of pyrenoids. There is a single nucleus with many nucleoli. There is well defined chromatin within the nucleus.

Internodal cell:

There is a large central vacuole. The cytoplasm is limited to peripheral layer. The cytoplams is not stationary but always in perpetual rotary motion. The protein bodies are dispersed in the cell sap. The discoid chloroplasts having no pyrenoids are embedded in the peripheral cytoplasm. There are many nuclei. The nuclei have no centrosomes.

The cell wall:

The cell wall consists of homogeneous cellulose. It is not multilayered. Outer to the cellulose wall there is a gelatinous layer, and this is the sheath for the deposition of calcium.

Reproduction:

The reproduction takes place by vegetative and sexual methods. Asexual reproduction is not found.

1. Vegetative reproduction:

The vegetative reproduction takes place by (a) tubers; (b) amylum stars and (c) secondary protomema.

(a) By tubers and bulbils:

The tubers are commonly formed on rhizoids or sometimes even on buried nodes. The whole structure is full of starch.

Sometimes the globule divides and becomes multicellular and known as ‘simple tuber’. When the tuber appears on the node, some of the peripheral cells go on dividing and massive structure is developed. Each starch filled tuber and bulbil may develop into a separate plant.

(b) By amylum or starch stars:

The cells of some subterranean nodes become star-shaped and very much laid in by starch are called amylum stars. Each such structure develops into a new plants.

(c) By secondary protonema:

The protonema like outgrowths come out from a node. Each such outgrowth is capable to develop into a new plant.

2. Sexual reproduction:

The sexual reproudction is oogamous. A very advanced and specialized type of oogamy is found. There is a special terminology for the sex organs. The male fruiting body is called globule and the female nucule.

Most of the species are homothallic (monoecious) and few are heterothallic (dioecious). The globule is borne on secondary lateral of limited growth. One globule and one nucule bome on one node of the leaf. In homothallic species both the fructifications borne on the same node. In Chara the nucule is borne above the globule.

Development of globule:

A single superficial nodal cell of the adaxial side of the leaf acts as the initial of both the fructifications, i.e., nucule and globule. This superficial cell divides into two derivatives by a transverse wall. One cell derivative of the superficial cell is the initial cell of the globule and the other is the initial cell of the nucule.

The globule initial cell divides transversely and two daughter cells are formed. The lower daughter cell does not divide further and converts into the pedicel cell. The upper daughter cell divides twice successively and four cells are formed arranged in quadrants. Each of these quadrants divides transversely and eight cells are produced thus attaining octant stage.

Each of these eight cells divides periclinally and thus produced eight outer cells which divide further periclinaily. The outermost eight cells are called shield cells. The middle cells are known as manubrial cells and the innermost eight cells are primary capitulum cells. The shield cells become very much enlarged and expanded.

The manubrial cells become very much radially elongated, but the primary capitulum cells are arranged compactly to each other in the centre of the globule. The outer walls of the shield cells fold inward and the shield cells appear multicellular structures. The infoldings are incomplete. The shield cells develop red pigments in them and so the globules appear orange red in colour.

From each primary capitulum cell six secondary capitulum cells are cut off inside the globule. These secondary capitulum cells rarely develop tertiary cells. On the secondary capitulum cells the initials of antheridial filaments are produced. These initials may also be produced upon primary or even tertiary capitulum cells.

Each antheridial initial develops into a branched or unbranched antheridial filament. Each antheridial filament has many compartments or cells in it. Each cell is supposed to be an antheridium. The protoplast of each antheridium metamorphoses into a single antherozoid. The antherozoid is elongated, coiled and biflagellate. The flagella are sub-terminal in origin. The nucleus is elongated and coiled. Some unused cytoplasm is found in the tail of the antherozoid.

On the maturity of the antherozoids the shield cells of the globule somewhat separate from each other, the antheridial filaments protrude out through these openings and the antherozoids liberate in the water. The liberation of antherozoids usually takes place in the morning.

Morphology of globule:

According to Hofmeister and Goebel the globule is a compound structure comprising of a large number of one-ceiled antheridia arranged in uniseriate, branched or unbranched antheridial filaments. According to these biologists, the antheridia of Charales are one-celled and at the par with the antheridia of other Thallophyta.

Development of nucule:

The nucule develops from the adaxial cell of basal node of the globule. The globule is homologous with the branch of limited growth and the nucule with the branch of unlimited growth.

The nucule initial divides twice and a row of three cells is formed. The terminal cell acts as oogonial mother cell which elongates sufficiently in vertical direction and transverse wall develops in the lower region of it dividing it into two cells. The lower small cell and the upper one is oogonium which contains an egg.

The lowermost cell of the row of three cells does not divide and acts as a pedicel. The middle cell divides vertically in such a way so that a single central cell and five sheath initials are produced. The sheath initials surround the central cell. The sheath initials elongate vertically sometimes even before the vertical elongation of the oogonial mother cell and encircle it.

Each of the sheath initials divides transversely forming the upper tier of coronary cells and lower tier of tube cells. The tube cells elongate several times to their original length and become spirally coiled around the oogonium. The coronary cells do not elongate much and act collectively as the corona of the nucule.

Fertilization:

Prior to fertilization the elongated and twisted tube cells become separated from each other and five small slits are developed just below the corona. The swimming antherozoids around the nucule try to enter through these openings.

The flagella are withdrawn and one of the antherozoids penetrates the egg. The male nucleus travels downwards and fuses with the egg nucleus developing a diploid (2n) nucleus. This diploid nucleus situates in the bottom of the zygote. The zygote settles down in the mud, secretes a thick wall and germinates on the approach of favourable conditions.

The zygote and its germination:

In favourable conditions the zygote germinates. The diploid (2n) nucleus moves to the top of the zygote and divides meiotically producing four halpoid nuclei. Simultaneously a septum divides the zygote into two unequal cells. The small distal cell is lenticular cell and contains one functional nucleus in it. The remaining big cell is called storage cell; possessing three nuclei in it disintegrate very soon.

The outer wall of the ornamented zygote cracks and the lenticular cell exposes. The lenticular cell divides by a vertical wall giving rise to a protonematal initial and a rhizoidal initial. The protonematal initial develops into a primary protonema which later on differentiates into nodes and internodes. The rhizoidal initial gives rise to a colourless rhizoid having nodes and internodes.

From the lowermost node of the protonema the appendages are given out which develop into secondary protonema or rhizoids. From the second node of the protonema a whorl of appendages is given out. All the appendages except one develop into green filaments. The life cycle is of Haplontic type. All phases but zygote are haploid.

Systematic position:

Division-Charophyta; Class-Charophyceae; Order-Charales; Family- Characeae; Genus – Chara.

Advanced features of the Charales:

The position of the Charales is controversial on account of the multicellular female organ, the complex antheridium, strong apical growth by vegetative shoots and a degree of specialization which is generally not found in other green algae. The female sex organ possesses a sterile jacket of cells, which is not found in the typical oogonium of other green algae. In 1875, Sachs referred the oogonium of Chara as a nucule.

The presence of sterile jacket of cells is a new thing for the algae whereas on the other hand it is characteristic of the archegonium in the Embryophyta. But the jackets of these two groups are not supposed to be homologous. A parallel or analogous situation is found in the Rhodophyceae, where the female organ is a fairly complex structure.

In the same way the antheridium of the Charales is a much more complex structure than the typical antheridia and does not resemble the antheridia of either Bryophyta or Tracheophyta. Sachs (1875) called the antheridium as a globule.

The Charales have also achieved a degree of specialized apical growth which superficially resembles that of the Equisetales. However, the Charales are devoid of vascular system and they cannot be compared with Equisetales.

The Charales are a unique group so much so that Smith and others have placed this order in a separate class, the Charophyceae. The complex sex organs, i.e., oogonia (nucule) and antheridia (globule) make a definite classification of the group difficult. However, the cellulose wall, pigmentation and food reserves are clearly of Chlorophycean type and according to Dr. Fritsch the order has been included is Chlorophyceae.